In general, a liquid crystal display (LCD) device needs to have a power supply circuit installed therein, for converting an external alternating current (AC) voltage into a direct current (DC) voltage. A typical power supply circuit is shown in FIG. 4. The power supply circuit 10 includes two input terminals 111 and 112, an output terminal 150, a first commutating and filter circuit 11, a transformer 14, a second commutating and filter circuit 15, a pulse width modulation circuit 16, a transistor 17, and a current limiting resistor 170.
The first commutating and filter circuit 11 includes a full-bridge rectifier circuit 113 and a first filter capacitor 114. The full-bridge rectifier circuit 113 includes two input terminals (not labeled), a positive output terminal (not labeled), and a negative output terminal (not labeled). The two input terminals of the full-bridge rectifier circuit 113 are connected to the two input terminals 111 and 112, respectively. The positive output terminal of the full-bridge rectifier circuit 113 is connected to ground via the first filter capacitor 114. The negative output terminal of the full-bridge rectifier circuit 113 is directly connected to ground.
The transformer 14 includes a primary winding 141 and a secondary winding 142. The primary winding 141 includes two taps (not labeled). One of the taps of the primary winding 141 is connected to the positive output terminal of the full-bridge rectifier circuit 113, and the other tap of the primary winding 141 is connected to a source electrode of the transistor 17. The secondary winding 142 also includes two taps (not labeled).
The second commutating and filter circuit 15 includes a first resistor 151, a first capacitor 152, a first diode 153, a second diode 154, a second filter capacitor 155. The first resistor 151 and the first capacitor 152 are connected in series between a first one of the taps of the secondary winding 142 and the output terminal 150. An anode of the first diode 153 is connected to the first tap of the secondary winding 142, and a cathode of the first diode 153 is connected to the output terminal 150. An anode of the second diode 154 is also connected to the first tap of the secondary winding 142, and a cathode of the second diode 154 is connected to the output terminal 150. The second tap of the secondary winding 142 is connected to ground. The output terminal 150 is connected to ground via the second filter capacitor 155.
The pulse width modulation circuit 16 includes a control port 161. The control port 161 is used to output a high level signal or a low level signal to turn on or turn off the transistor 17.
A gate electrode of the transistor 17 is connected to the control port 161 of the pulse width modulation circuit 16. A drain electrode of the transistor 17 is connected to ground via the current limiting resistor 170.
The external AC voltage is inputted to the two input terminals 111 and 112. The AC voltage is converted into a direct current (DC) voltage via the first commutating and filter circuit 11.
When the transistor 17 is turned on, the first filter capacitor 114, the primary winding 141 of the transformer 14, the transistor 17, and the current limiting resistor 170 form a first circuit path (not labeled). The first filter capacitor 114 can be regarded as a power source, and the primary winding 141 can be regarded as an inductor. A current flowing through the primary winding 141 linearly increases until the current reaches a maximum value when a voltage of the first filter capacitor 114 is constant. Such voltage can be expressed by the following equation (1):
                    V        =                  L          ⁢                                    ⅆ              I                                      ⅆ              t                                                          (        1        )            where V represents the voltage of the first filter capacitor 114, L represents an inductance of the primary winding 141, I represents the current flowing through the primary winding 141, and t represents time.
When the transistor 17 is turned off, electrical energy stored in the primary winding 141 is transmitted to the secondary winding 142, and is then converted into a steady DC voltage via the second commutating and filter circuit 15. Then the steady DC voltage is outputted to circuits in other parts of the LCD device via the output terminal 150.
However, in practice, the external AC voltage may increase suddenly. When this happens, electrical energy stored in the primary winding 141 and the secondary winding 142 of the transformer 14 may increase significantly. The voltage outputted by the second commutating and filter circuit 15 correspondingly increases significantly. Thus, the output terminal 150 may apply a large voltage to the circuits in the other parts of the LCD device. The large voltage is liable to disrupt normal operation of the LCD device.
Furthermore, when the external AC voltage increases suddenly, the current flowing through the first circuit path and a current flowing through the second commutating and filter circuit 15 correspondingly increase significantly. If any electronic component of the power supply circuit 10 is thereby impaired or damaged, the whole power supply circuit 10 is liable to be burned out.
Accordingly, what is needed is a power supply circuit for LCD devices that can overcome the above-described deficiencies.